Glass fibers are commonly utilized in many industrial and consumer products throughout the world. Consequently, it is important that the production of such glass fibers be accomplished in an efficient fashion. In one known machine for producing these glass fibers, a precious metal bushing is provided having an internal cavity and a bottom portion. The internal cavity forms a reservoir adapted to receive molten glass therein. The bottom portion has a plurality of relatively small apertures formed therethrough which extend into the internal cavity. The molten glass in the internal cavity is drawn through the apertures to form the individual glass fibers. The bushing is heated to a relatively high temperature to facilitate the flow of the molten glass through the apertures.
As the individual glass fibers are produced, they are usually combined together into multiple fiber strands. The strands are then wound upon respective cylindrical tubes, each of which is rotated by a mechanical winder. A winder is usually provided for each bushing. The rotation of each tube by its corresponding winder draws the molten glass through the apertures under tension and, therefore, attenuates the glass fibers and strands as they are formed. Preferably, a predetermined uniform length of each strand is wound upon each tube before the wound tube is removed from the winder and a bare tube is inserted. In order to achieve this predetermined uniform strand length on each tube, it is important that the glass fibers be produced having relatively uniform diameters throughout their length.
The diameter of each of the glass fibers (and, consequently, of each of the strands) produced by the machine described above is dependent upon several factors. One such factor is the amount of the mechanical attenuation induced in the glass fibers and strands by the winders. Such mechanical attenuation is caused by the pulling of the glass fibers and strands by the rotating tubes. In order to maintain the diameters of the strands constant, the amount of molten glass which passes through the bushing apertures per unit time should be constant. Assuming that the temperature of the bushing is fixed at a certain level, the viscosity of the molten glass will remain constant. Therefore, the diameters of the glass fibers and strands will remain constant so long as the linear speed at which the molten glass is drawn through the bushing apertures does not change.
However, as each glass strand is wound about its tube, the effective outer diameter of the tube is increased by the continuous overlapping of the strand upon itself. If the winder is driven at a constant rotational speed, therefore, the tangential speed at which the strand is wound about the tube (and, therefore, the linear speed at which the glass fibers are drawn through the bushing apertures) will increase with the effective outer diameter of the tube. To compensate for this phenomenon, variable rotational speed winders have been provided, which reduce the rotational speed of the tube as it is wound with the glass strand. As a result, the linear speed at which the glass fibers are drawn through the bushing apertures is maintained relatively constant.
A temperature controller is commonly utilized to regulate the temperature of the bushing. In one such temperature controller which is known in the art, a power supply causes an electric current to pass through the bushing. Since the bushing resists the flow of such electric current, the amount of heat generated in the bushing is proportional to the amount of the electric current passed therethrough. The amount of the current is controlled in response to a predetermined desired temperature signal. One or more thermocouples may be attached to various locations on the bushing. The thermocouples generate electrical signals which are representative of the temperature of the bushing at the attached locations. Those electrical signals are fed back to a temperature comparator, where they are compared with the desired temperature signal. If the actual temperature of the bushing is less than the desired temperature, the temperature comparator causes the power supply to increase the amount of electric current passing through the bushing, and vice versa. As a result, a predetermined desired temperature of the bushing is achieved. This temperature feedback provides an additional measure of accuracy for the temperature controller. By providing a plurality of such thermocouples, the temperature of each of a plurality of individual sections of the bushing can be controlled independently of one another.
Unfortunately, the variable rotational speed winders described above are not always available for use with the glass fiber forming machines. Thus, an alternative approach is required for constant speed winders in order to prevent changes in the mechanical attenuation of the glass fibers from occurring during the winding process. One alternative approach which is known in the art involves increasing the temperature of the bushing as the glass strand is wound upon the tube. As the temperature of the bushing is increased, the viscosity of the molten glass retained therein is decreased, and the molten glass flows more freely through the bushing apertures. Therefore, even though the linear speed at which the glass fibers are drawn through the bushing apertures increases as a result of the increased effective outer diameter of the tube being wound, the amount of the molten glass which passes through the bushing apertures per unit time remains relatively constant. Thus, the amount of the mechanical attenuation (and, therefore, the diameter) of the glass fibers and strands remains approximately constant.
Generally, if the temperature of the bushing is increased at a constant linear rate over the period of time required to completely wind the tube, the resulting diameters of the glass fibers and strands will remain relatively constant. To accomplish this, it is known to modify the temperature feedback signal from the thermocouples with an electrical signal from a common resistor-capacitor temperature compensating circuit. The capacitor is charged at a rate determined by the values of the resistor and the capacitor. As a result, the voltage differential across the capacitor increases over time at a predetermined rate. The values of the resistor and the capacitor are selected such that the range of the voltage differential across the capacitor is approximately linear for the required period of time. The temperature compensating circuit signal is combined with the actual temperature feedback signal to generate a composite feedback signal. The composite feedback signal indicates to the temperature comparator that the temperature of the bushing is decreasing below the actual temperature thereof as the tube is wound. Because of this, the temperature comparator causes the power supply to increase the amount of current being passed through the bushing, even though the actual temperature of the bushing is not less than the desired temperature. This increase in current is controlled in accordance with the value of the voltage differential across the capacitor. When the tube is completely wound, the capacitor is discharged, and the process described above is repeated.
The resistor-capacitor temperature compensating circuit described above is undesirable for several reasons. First, the charging of the capacitor often occurs at a non-linear rate because of variations in the source of voltage utilized to operate the system and because the timing circuit is sensitive to ambient temperature and humidity conditions. Second, the initial set up of such a circuit, including the precise setting of the starting and ending current values, is difficult to achieve and, in any event, is time consuming. Accordingly, it would be desirable to provide an improved method and apparatus for varying the temperature of the bushing in a glass fiber forming machine having a constant speed winder.